High strength military steel

ABSTRACT

A low cost high hardness, high strength, and high impact toughness steel for military articles such as armor plates, bodies of deep penetrating bombs, and missiles. After oil quenching, refrigerating, and low tempering, the new steel consisting of (% wt.): C=0.35, Cr=1.32, Mo=0.35, W=0.52, Ni=2.66, Mn=0.85, Cu=0.51, Si=0.83, V=0.26, Ti=0.12, and a balance of Fe and incidental impurities has a HRC of 55, UTS of 301 ksi, YS of 233 ksi; and Charpy V-notch impact toughness energy of 26 ft-lbs.

RELATED APPLICATIONS

This application claims the benefit of priority of U.S. provisional patent application No. 61/128,189, filed May 20, 2008, which is hereby incorporated by reference in its entirety.

FIELD OF THE INVENTION

This invention relates to a low cost high hardness, high strength, high impact toughness military steel and more particularly to a military steel with higher mechanical performance than Eglin steel.

BACKGROUND OF THE INVENTION

Large amounts of expensive high hardness, high strength, and high impact toughness military steels are used for purposes such as bunker buster bombs, missiles, tank bodies and aircraft landing gears.

Eglin Steel was a joint effort of the US Air Force and Ellwood National Forge Company program to develop a low cost replacement for the expensive high strength and high toughness steels, AF-1410, Aermet-100, HY-180, and HP9-4-20/30. One application of Eglin steel was the new bunker buster bombs, e.g. the Massive Ordnance Penetrator and the improved version of the GBU-28 bomb known as EGBU-28.

High strength is required to survive the high impact speeds that occur during deep penetration. Eglin steel was planned for a wide range of other applications, from missile and tank bodies to machine parts.

One shortcoming of Eglin steel is its limited mechanical properties for large manufactured products which are as follows:

-   -   Hardness (HRC), up to C48     -   Ultimate tensile strength (UTS), up to 250 ksi     -   Yield strength (YS) up to 210 ksi

Another shortcoming of Eglin steel is that its structural performance during impact tests of large articles, such as bunker buster bombs, vary somewhat below the impact test results of smaller laboratory products. The discrepancies in results are due to difficulties with heat treating of Eglin steel.

The present invention overcomes the shortcomings of Eglin steel by providing a lower cost steel that has higher mechanical properties and consistent results from heat treating. The improved steel has a medium carbon content, low nickel, molybdenum, and tungsten contents, and the strong carbide forming elements vanadium and titanium or niobium. The new alloying concentrations of vanadium, titanium or niobium, and tungsten affect the conditions of melting, processing, and heat treatment and as a result, it's higher mechanical properties.

One benefit of the new steel is higher performances of armor plate, deep penetrating bombs and missiles. Another benefit is that, at the same performance, less steel is required to match the performance of Eglin steel.

Another benefit of the invention is that smaller amounts are required of the expensive elements nickel (Ni) and tungsten (W). The invention requires at most 3.5% of Ni and 2% of W, versus of 5% of Ni and 3.25% of W for Eglin Steel.

SUMMARY OF THE INVENTION

The present invention is a lower cost military steel (“new steel”) with higher levels of hardness, strength, and impact toughness than Eglin steel. The higher mechanical properties are due to optimizations of the following factors:

-   -   selections of alloying compositions that supply high hardness,         strength, and impact toughness     -   selections of critical temperatures

The hardness, strength and impact toughness of the invention was verified by the melting of laboratory and industrial scale ingots, processing of ingots from the melt, production of articles from the ingots, heat treating of the articles and mechanical testing of the articles.

The new steel differs from Eglin Steel by the following features:

-   -   A microstructure of tempered dispersed lath martensite         consisting of small packets of martensite laths grown on fine         carbides and retained austenite, and packet boundaries free of         carbides after quenching, low tempering or quenching,         refrigerating, and low tempering.     -   After quenching and low tempering, a Rockwell hardness of         C52-54, an ultimate tensile strength of 285-295 ksi, a yield         strength of 215-220 ksi, an elongation of 13-14%, a reduction of         area of 48-50%, and a Charpy V-notch impact toughness energy of         26-30 ft-lb.     -   After quenching, refrigerating, and low tempering, a Rockwell         hardness of C54-56, an ultimate tensile strength of 290-305 ksi,         a yield strength of 225-235 ksi, an elongation of 13-14%, a         reduction of area of 47-50%, and a Charpy V-notch impact         toughness energy of 26-28 ft-lb.     -   After quenching and a second hardening by high tempering a         microstructure consisting of a fine dispersion of titanium         carbide (TiC), vanadium carbide (VC), and complex tungsten         carbides, (MW)_(x)C_(y) in a ferritic-martensitic-retained         austenite matrix     -   After quenching and a second hardening by high tempering, a         Rockwell hardness of C 48-50, an ultimate tensile strength of         240-250 ksi, a yield strength of 225-235 ksi, an elongation of         10-11%, a reduction of area of 48-50%, and a Charpy V-notch         impact toughness energy of 20-22 ft-lb     -   A high ductility and high formability during hot forging or         rolling     -   A use of only homogenized and recrystallization annealing         without normalizing for the low tempered new steel     -   A sum of alloying elements of that is less than the sum of         alloying elements of Eglin steel     -   Cost of charge materials of the new steel is less than cost of         charge materials of Eglin steel.

The chemical compositions and mechanical properties of the invention and Eglin steel are compared in FIG. 1 and FIG. 2.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 compares the chemical compositions of the new steel and Eglin Steel.

FIG. 2 compares the mechanical properties at room temperature of Eglin Steel and the invention after quenching and low tempering; after quenching, refrigerating, and low tempering; and after quenching and a second hardening by high tempering.

DETAILED DESCRIPTION OF THE INVENTION

The composition of the invention is comprised of: carbon (C); ferrite stabilizing chromium (Cr), molybdenum (Mo); silicon (Si); strong carbide forming tungsten (W), vanadium (V), and titanium (Ti) or niobium (Nb); austenite stabilizing nickel (Ni), manganese (Mn), copper (Cu); iron (Fe) and incidental impurities.

The carbon (C) content of 0.30 to 0.45% wt. supports the forming of carbides of tungsten (W), vanadium (V), titanium (Ti) or niobium (Nb), and complex carbides as centers of growth of martensite laths forming the microstructure of tempered dispersed lath martensite with retained austenite.

The chromium (Cr) content of 1.0 to 3.0% wt. increases strength, hardenability and temper resistance.

The molybdenum (Mo) content of 0.1 to 0.55% wt. improves hardenability, eliminates reversible temper brittleness, resists hydrogen attack & sulfur stress cracking, and increases elevated temperature strength.

The nickel (Ni) content of 0.1 to 3.5% wt. supplies impact toughness.

The manganese (Mn) is a strong deoxidizing, and austenite stabilizing element. It's content is 0.1 to 1.0% wt.

The silicon (Si) strengthens the steel matrix by increasing the bonds between atoms in a solid solution. It protects the grain boundary from the growth of carbides, which decrease the toughness of the new steel. The content of Si is 0.1 to 1.0% wt.

The copper (Cu) improves corrosion resistance, ductility, and machinability. The preferred content of Cu is 0.1 to 0.6% wt.

The tungsten (W) forms fine dispersed carbides, eliminates reversible temper brittleness, and increases hardness and temperature resistance. Its content is 0.1 to 2.0% wt.

The vanadium (V) affects on the structure and properties of the new steel in several ways. It forms finely dispersed particles of carbides in austenite which control the size and shape of grains by precipitating vanadium based, finely dispersed secondary carbides during high tempering and by affecting the kinetic and morphology of the austenite-martensite transformation. The concentration of V is 0.1 to 0.55% wt.

The titanium (Ti) and niobium (Nb) are more active carbide forming elements than vanadium (V). Small concentrations of the strong carbide forming titanium (Ti) or niobium (Nb) do not affect the kinetics of phase transformations. A basic function of these elements is to inhibit austenite grain growth at high temperatures during heating. One element Ti or Nb is a part of the new steels. The concentration of Ti or Nb is 0.02 to 0.2% wt.

The balance of the new steel is iron (Fe) and incidental impurities.

Industrial scale ingots of the new steel were initially melted in an open induction furnace and then were melted in an electro-arc furnace (EAF), utilizing scrap and conventional charge materials. From the EAF, the steel was transported to a ladle refining furnace (LRF). In LRF the steel was reheated, refined from impurities, the necessary ingredients were added, and the steel was homogenized. Thereafter, the steel was transported to a vacuum de-gas station to remove hydrogen and nitrogen. Liquid steel was poured at 2950 to 3000° F. into iron molds. Ingots were subjected to homogenized annealing at 2100 to 2150° F. for 12-24 hours. Afterwards, the ingots were heated to 2100 to 2150° F. and forged to final size blanks. The blanks were subjected to re-crystallization annealing at 1080 to 1150° F. for 8-18 hours. Some ingots were subjected to normalizing at 1925 to 1950° F. for 8-12 hours and high tempering at 1100 to 1120° F. for 8-12 hours to eliminate the banding microstructure after the severe hot forging.

After austenizing at 1875-1925° F. and further quenching and low tempering or quenching, refrigerating, and low tempering, a tempered martensite microstructure consisting essentially of martensitic lathes, fine titanium carbide, TiC as centers of growth of the martensitic laths, and retained austenite was formed. The boundaries of the packets were free of carbides.

The second hardening of the new steel by high tempering consists of heating at 950-1200° F. for 5-7 hours to precipitate vanadium carbide, VC and complex tungsten carbides, (MW)_(x)C_(y) as a fine dispersion.

After quenching and second hardening by high tempering, the new steel had a microstructure consisting of fine dispersion titanium carbide, TiC, vanadium carbide, VC, complex tungsten carbides, (MW)_(x)C_(y) in a ferritic-martensitic-retained austenite matrix.

True production cost of the new steel is difficult to assess. However, based on data of the London Metal Exchange (LME), dated April, 2009, cost of charge materials of the new steel is at most 3,150 USD per metric ton, versus of Eglin steel at most 3,850 USD per metric ton.

EXAMPLES OF THE NEW STEEL Example 1

The composition of the new steel is comprised of (%, wt): C=0.37, Cr=1.25, Ni=3.45, Mn=0.82, Cu=0.52, V=0.24, Si=0.91, Mo=0.52, Ti=0.11, and a balance of Fe and incidental impurities.

The new steel has the following critical temperatures, upper critical temperature A_(C3), low critical temperature A_(C1), and martensite start temperature M_(S):

A_(C3)=1465° F., A_(C1)=1260° F., M_(S)=440° F.

Processing of laboratory scale ingots of the new steel consists of:

-   -   Homogenized annealing at 2100° F. for 6 hrs and air cooling     -   Hot rolling with a start temperature of 2150° F. and a finish         temperature of 1850° F. and air cooling     -   Recrystallization annealing at 1100° F. for 4 hrs

Test specimens of the new steel are heat treated in the following manner:

-   -   Austenizing at 1900° F. for 60 min.     -   Oil quenching for 2.5 min. and further air cooled     -   Refrigerating at −60° F. for 60 min.     -   Tempering at 400° F. for 4 hrs.

The new steel has the following room temperature mechanical properties:

HRC UTS (ksi) YS (ksi) EL (%) RA (%) CVN (ft-lb) 54 296 234 14 50 27.5

The new steel has a tempered martensite microstructure consisting of martensitic laths, fine titanium carbides, TiC as centers of growth of the martensitic laths, and at most 14% of retained austenite. The boundaries of the packets are free of carbides.

Example 2

The composition of the new steel is comprised of (%, wt): C=0.35, Cr=1.32, W=0.52, Ni=2.66, Mn=0.85, Cu=0.51, V=0.26, Si=0.83, Mo=0.35, Ti=0.12, and a balance of Fe and incidental impurities.

The new steel has the following critical temperatures:

A_(C3)=1475° F., A_(C1)=1270° F., M_(S)=485° F.

Laboratory scale ingots of the new steel are processed the same as Example 1.

Test specimens of the new steel are heat treated in the following manner:

-   -   Austenizing at 1900° F. for 60 min.     -   Oil quenching for 2.5 min. and further air cooled     -   Refrigerating at −60° F. for 60 min.     -   Tempering at 450° F. for 4 hrs.

The new steel has the following room temperature mechanical properties:

HRC UTS (ksi) YS (ksi) EL (%) RA (%) CVN (ft-lb) 55 301 233 13.5 49 26

The microstructure of the new steel is similar to the microstructure of Example 1 and has a retained austenite at most 11% wt.

Example 3

The composition of the new steel is comprised of (%, wt): C=0.32, Cr=1.24, W=0.82, Ni=2.52, Mn=0.86, Cu=0.53, V=0.25, Si=0.87, Mo=0.38, Ti=0.11, balance essentially Fe.

The new steel had the critical temperatures:

A_(C3)=1470° F., A_(C1)=1265° F., M_(S)=455° F.

Laboratory scale ingots of the new steel had the same processing as in Example 1.

Test specimens of the new steel was heat treated by the following mode:

-   -   Austenizing at 1900° F. for 60 min.     -   Oil quenching for 2.5 min. and further air cooled     -   Refrigerating at −60° F. for 60 min.     -   Tempering at 420° F. for 4 hrs.

The new steel has the following room temperature mechanical properties:

HRC UTS (ksi) YS (ksi) EL (%) RA (%) CVN (ft-lb) 55 298 229 13.5 49 26

The new steel has a microstructure that is similar to the microstructure of Example 1 and has a retained austenite at most 9% wt.

Example 4

The composition of the new steel is comprised of (%, wt): C=0.37, Cr=1.61, Ni=0.54, Mn=0.41, Cu=0.29, V=0.54, Si=0.75, Mo=0.49, W=1.23, Ti=0.11, and a balance of Fe and incidental impurities.

The new steel has the following critical temperatures:

A_(C3)=1555° F., A_(C1)=1345° F., M_(S)=565° F.

Processing of laboratory scale ingots of the new steel is comprised of:

-   -   Homogenized annealing at 2100° F. for 6 hrs and air cooling     -   Hot rolling with a start temperature of 2150° F. and a finish         temperature of 1850° F. and air cooling     -   Recrystallization annealing at 1150° F. for 4 hrs     -   Normalizing at 1925° F. for 4 hrs

Test specimens of the new steel was heat treated by the following mode:

-   -   Austenizing at 1900° F. for 60 min.     -   Oil quenching for 2.5 min. and further air cooled     -   Second hardening by high tempering at 1070° F. for 3 hrs. and         further high tempering at 1000° F. for 4 hrs.

The new steel has the following room temperature mechanical properties:

HRC UTS (ksi) YS (ksi) EL (%) RA (%) CVN (ft-lb) 49 250 234 10 49 20.5

The new steel has a microstructure that consists essentially of a fine dispersion of titanium carbide, TiC, vanadium carbide, VC, complex tungsten carbides, (MW)_(x)C_(y) in a ferritic-martensitic-retained austenite matrix.

Example 5

The composition of the new steel is comprised of (%, wt): C=0.35, Cr=1.43, Ni=0.69, Mn=0.43, Cu=0.31, V=0.52, Si=0.72, Mo=0.52, W=1.35, Ti=0.12, and balance essentially Fe.

The new steel has the following critical temperatures:

A_(C3)=1560° F., A_(C1)=1345° F., M_(S)=580° F.

Laboratory scale ingots of the new steel are processed the same as Example 4.

Test specimens of the new steel are heat treated in the same manner as Example 4.

The new steel has the following room temperature mechanical properties:

HRC UTS (ksi) YS (ksi) EL (%) RA (%) CVN (ft-lb) 49 249 234 10 48 21

The new steel has a microstructure that is similar to the microstructures of Example 4.

From the above, it is apparent that the high hardness, high strength, high impact toughness steel which is the subject of the invention is an important development in the steel making art. Although only five examples have been described, it is obvious that other examples of the new steel can be derived from what is claimed in the presented description without departing from the spirit thereof. 

1. A low cost high hardness, high strength, and high impact toughness steel comprising by weight about: 0.3 to 0.45% of C; 0.1 to 3.0% of Cr; 0.1 to 0.55% of Mo; 0.1 to 2.0% of W; 0.1 to 3.5% of Ni; 0.1 to 1.0% of Mn; 0.1 to 1.0% of Si; 0.1 to 0.6%% of Cu; 0.02 to 0.2% of Ti or Nb; 0.1 to 0.55% of V; and a balance of Fe and incidental impurities.
 2. The low cost steel recited in claim 1 wherein after quenching and low tempering, said steel has a hardness of Rockwell C52 to 54, an ultimate tensile strength of 285 to 295 ksi, a yield strength of 215 to 220 ksi, an elongation of 13 to 14%, a reduction of area of 48 to 50%, and Charpy v-notch impact toughness energy of 26 to 30 ft-lb at the room temperature.
 3. The low cost steel recited in claim 1 wherein after quenching, refrigerating and low tempering, said steel has a hardness of Rockwell C54 to 56, an ultimate tensile strength of 290 to 305 ksi, a yield strength of 225 to 235 ksi, an elongation of 13 to 14%, a reduction of area of 47 to 50%, and Charpy v-notch impact toughness energy of 26 to 28 ft-lb at the room temperature.
 4. The low cost steel recited in claim 1 wherein after quenching and a second hardening by high tempering, said steel has a hardness of Rockwell C48 to 50, an ultimate tensile strength of 240 to 250 ksi, a yield strength of 225 to 235 ksi, an elongation of 10 to 11%, a reduction of area of 48 to 50%, and Charpy v-notch impact toughness energy of 20 to 22 ft-lb at the room temperature.
 5. A low cost high hardness, high strength, and high impact toughness steel has a fine dispersed tempered martensite microstructure comprised of small packets of martensitic laths, fine titanium carbides as centers of growth of said martensitic lathes, and retained austenite; boundaries of said packets are free of carbides; said steel comprised of by % weight about: 0.3 to 0.45% of C; 0.1 to 2.0% of W; 0.1 to 0.55% of V; 0.02 to 0.2% of Ti or Nb; at most 9.65% of sum of Cr, Mo, Ni, Mn, Si, Cu; and a balance of Fe and incidental impurities
 6. The low alloy steel recited in claim 5 wherein said steel comprises by weight of about: 0.37% of C; 1.25% of Cr; 0.51 of Mo; 0.51% of W; 3.45% of Ni; 0.82% of Mn; 0.91 of Si; 0.52% of Cu; 0.24% of V; 0.11% of Ti, and a the balance essentially Fe and incidental impurities, and after austenizing at about 1900 F, oil quenching, refrigerating at about −60 F, and tempering at about 400 F, said steel has a hardness of about Rockwell C 54, an ultimate tensile strength of about 296 ksi, a yield strength of about 234 ksi, and Charpy v-notch impact energy of about 27.5 ft-lb at the room temperature.
 7. The low alloy steel recited in claim 5 wherein said steel comprises by of about: 0.35% of C; 1.32% of Cr; 0.35 of Mo; 0.52% of W; 2.66% of Ni; 0.83% of Mn; 0.85 of Si; 0.51% of Cu; 0.26% of V; 0.12% of Ti; and a balance essentially of Fe and incidental impurities, and after austenizing at about 1900 F, oil quenching, refrigerating at about −60 F, and tempering at about 450 F, said steel has a hardness of about Rockwell C 55, an ultimate tensile strength of about 301 ksi, a yield strength of about 233 ksi, and Charpy v-notch impact energy of about 26 ft-lb at the room temperature.
 8. The low alloy steel recited in claim 5 wherein said steel comprises by weight of about: 0.32% of C; 1.24% of Cr; 0.38 of Mo; 0.82% of W; 2.52% of Ni; 0.86% of Mn; 0.87 of Si; 0.53% of Cu; 0.25% of V; 0.11% of Ti; a balance of Fe and incidental impurities and after austenizing at about 1900 F, oil quenching, refrigerating at about −60 F, and low tempering at 420 F, said steel has a hardness of about Rockwell C 55, an ultimate tensile strength of about 298 ksi, a yield strength of about 229 ksi, and Charpy v-notch impact toughness energy of about 26 ft-lb at the room temperature.
 9. A low cost high strength steel has a microstructure comprised of a fine dispersed titanium, vanadium, and complex tungsten carbides in a ferritic-martensitic-retained austenite matrix; said steel comprising by weight of about: 0.3 to 0.45% of C; 0.1 to 2.0% of W; 0.1 to 0.55% of V; 0.02 to 0.2% of Ti or Nb; at most 9.65% of sum of Cr, Mo, Ni, Mn, Si, Cu; and a balance of Fe and incidental impurities
 10. The low cost steel recited in claim 9 wherein said steel comprises by weight of about: 0.37% of C; 1.61 of Cr; 0.49 of Mo; 1.23% of W; 0.54% of Ni; 0.41% of Mn; 0.75 of Si; 0.29% of Cu; 0.54% of V; 0.11% of Ti; 1.61 of Cr; and a balance of Fe and incidental impurities, and after austenizing at about 1900 F, oil quenching, tempering at 1070 F followed by tempering at 1000 F, said steel has a hardness of about Rockwell C 49, an ultimate tensile strength of about 250 ksi, a yield strength of about 234 ksi, and Charpy v-notch impact toughness energy of about 20.5 ft-lb at the room temperature.
 11. The low cost steel recited in claim 9 wherein said steel comprises by weight of about; 0.35% of C; 1.43 of Cr; 0.52 of Mo; 1.35% of W; 0.69% of Ni; 0.43% of Mn; 0.72 of Si; 0.31% of Cu; 0.52% of V; 0.12% of Ti; and a balance of Fe and incidental impurities and after austenizing at about 1900 F, oil quenching, tempering at about 1070 F followed by tempering at about 1000 F, said steel has a hardness of about Rockwell C 49, an ultimate tensile strength of about 249 ksi, a yield strength of about 234 ksi, and a room temperature Charpy v-notch impact toughness energy of about 21 ft-lb at the room temperature. 